The present invention relates to a means for preventing toner remaining on an image carrier after the image carrier is cleaned by a cleaning device of an image forming apparatus such as a copying machine or a printer.
An electrophotographic color image forming apparatus such as a copy is generally comprises a platen on which a document is placed, a scanning optical system composed of a document-illuminating lamp reciprocating under the platen, a reflection mirror, a fixed projection lens and a reflection mirror, a photoreceptor drum which is charged by a charging electrode to a certain level of voltage and rotates, a developing unit which superposes a plurality of color toners on an electrostatic latent image formed on the surface of the photoreceptor drum by the scanning optical system for developing a multicolor image, a transfer electrode which transfers a toner image onto a recording sheet that is a transfer material conveyed toward the lower part of the photoreceptor drum surface, a separating electrode and a neutralizing electrode, a fixing unit which fixes the toner image transferred onto the recording sheet, an ejecting unit for the transfer material, and a cleaning unit that removes toner and dust remaining on the photoreceptor surface after transferring.
Due to the foregoing, necessary color images are recorded in succession on transfer materials conveyed one by one intermittently along the predetermined conveyance path.
After that, the photoreceptor drum cleaned by the cleaning unit is charged through a predetermined image process to be ready for the following image forming cycle.
A cleaning blade of the cleaning unit in a color image forming apparatus is away from a photoreceptor drum while a multicolor image is being formed on the circumferential surface of the photoreceptor drum, and the blade is brought into contact with the circumferential surface of the photoreceptor drum again after the necessary color image is formed and transferred onto a transfer material for cleaning residual toner. After the cleaning blade has completed a necessary cleaning by scraping off toner, when the cleaning blade leaves the photoreceptor drum, a part of toner collected on the edge of the cleaning blade remains on the circumferential surface of the photoreceptor drum in the form of a streak.
When residual toner just adhering to the photoreceptor drum arrives at the developing unit in the course of a rotation of the photoreceptor drum, and passes through a relatively narrow gap of about 500 .mu. between the surface of the photoreceptor drum and a developing sleeve of each rotating developing unit, the residual toner enters the developing unit and is mixed therein. It does not cause any problem in a monochromatic image forming apparatus. In a color image forming apparatus, however, yellow toner in the developing unit, for example, is mixed with the residual toner whose color is different from that of the yellow toner, resulting in deterioration of a color tone of the yellow toner and an adverse effect on color images to be developed.
An object of the invention is to solve the problem mentioned above and to provide an image forming apparatus wherein, even when toner remains on a photoreceptor drum of a color image forming apparatus that forms multicolor images after cleaning, the toner does not enter a developing unit so that normal color images can be formed.